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Transformation induced plasticity (TRIP) steels are a promising solution for the production of cars with low body mass because of the combination of high strength and large uniform elongation that they offer. However, conventional CMnSi TRIP steels with more than 1 wt-%Si have the drawback of poor Zn coating quality after continuous galvanising. This problem is due to the presence of complex Si ± Mn oxides on the strip surface. The present research work therefore focused on the full substitution of Si by Al in TRIP steels and the detailed analysis of the galvanising behaviours of these Si free CMnAl TRIP steels. If the hot dipping is done after a combination of intercritical annealing and isothermal bainitic transformation in a furnace atmosphere with a high dewpoint, the wetting of the strip by the liquid Zn is improved signi® cantly. However, the improvement is limited and not enough to avoid bare spots and coating defects cannot be avoided on conventional CMnSi TRIP steel. In contrast, the Si free CMnAl TRIP steel has a much better wettability when annealed at a low dewpoint. The surface segregation of the elements, which have a high af® nity for oxygen, i.e. Si, Al, and Mn, was studied in detail and this revealed that Si was much more readily enriched on the surface than Al during the annealing in the low dewpoint atmosphere. The difference in the surface segregation between Si and Al resulted in a clear difference in the galvanisability. The limited presence of Al on the strip surface is due to the fact that Al can be oxidised internally during hot rolling. As a result, an Al depleted surface region is formed owing to selective internal oxidation of Al before the continuous galvanising.MST/5293

A high-strength interstitial-free (HS IF) steel, solution strengthened with P and Mn, was hot-dip galvanized and galvannealed in a laboratory hot-dip simulator. The dew point of the N 2 -H 2 annealing atmosphere during recrystallization was varied, and the effect of the dew-point changes on the galvannealing kinetics was studied. Annealing in a high-dew-point N 2 -H 2 atmosphere improved the wettability by the zinc and the galvannealing reactivity of the Mn-P-added IF steels considerably. Scanning electron microscopy (SEM) analysis and glow discharge optical emission spectroscopy (GDOES) depth profiles of the surface of the steel sheet and the subsurface region revealed clear differences in oxidation mode after recrystallization annealing. Plane-view SEM and transmission electron microscopy (TEM) analyses were done on the inhibition layer of the as-galvanized samples. Differences in the Mn and P contents were found. By analyzing the Mn and P contents in the coatings during galvannealing, a mechanism was proposed to explain the higher galvannealing reactivity which is observed when the dew point during recrystallization annealing is increased.

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